It's not a great title to carry. Being the rarest of anything indicates that you may well soon disappear altogether.

On first sight, prospects do look grim for the Hainan gibbon (Nomascus hainanus). These apes are not only the rarest primate in the world, they are the rarest mammal alive today.

Half a century ago we didn't know much about the Hainan gibbon

There are thought to be as few as 26 individuals remaining, and they all live in a small part of the rainforest in China's Hainan province, an island off the coast of southern China.

Although protected, their habitat shows sign of gradual decline as locals scour the forest for edible or medicinal plants, occasionally hunting the other animals that share the gibbons’ environment. 

Half a century ago we didn't know much about the Hainan gibbon. There were about 2,000 individuals at that point, but it wasn’t clear whether they actually constituted a unique species. By the time we learned that they do, over-hunting and logging in their forests had reduced their numbers dramatically. But new insights into their lives may yet help save this rare, beautiful ape.  

There are 17 species of gibbon, but the Hainan gibbon is rather special, for more reasons than its rarity. 

Within the past 20 years primatologists have realised that these gibbons are not only a distinct species but also the most genetically distinct gibbon alive. We now also know that they behave quite unlike the others.

 All the gibbons are isolated in a single forest patch within that wider landscape

Most gibbons have a social structure that consists of an adult male and female, with several juveniles. The Hainan gibbon bucks this trend: one adult male lives with several females. 

What wasn’t clear was whether this is the way Hainan gibbons have always lived or whether their unusual social structure is a result of their devastatingly low population. Samuel Turvey of the Zoological Society of London, has been observing this group for many years. He concludes that their unusual social structure is normal for them. And that knowledge is vital if any future conservation programme is to be successful. 

 

For example, one idea is to save the Hainan gibbons by helping them move to other parts of the forest. If they were transported into other habitats in the wrong groupings, it might be devastating for their future prospects. 

Though they live in a reasonably large protected area, the forest that they spend most of their time in is quite fragmented. "All the gibbons are isolated in a single forest patch within that wider landscape," says Turvey. "There are areas of forest even there which gibbons aren’t occupying."

 If they are not in a social group, they rarely sing

Before any effort to move them into other parts of the reserve, conservationists would like to see their numbers increase. But it seems their population level has hit a mysterious plateau. Though they are reproducing at a normal level for these apes, their numbers have failed to rise. 

Turvey has some ideas why this may be the case. As individuals become sexually mature, they typically branch away from their family unit. They should then form a new group and create a family of their own. But some individuals don't: they roam the forests alone. This makes them extremely hard to monitor, says Turvey. "If they are not in a social group, they rarely sing - our understanding of what happens to [the lone individuals] is anecdotal." 

We don’t know why some of the gibbons are living alone. It might be because the limited size of their habitat can’t support any more groups, or it could be due to human disturbances. Though humans do not hunt them, they do hunt other animals in their forest.

The loners might also fail to form new groups because their family ties have become too close. Their genetic similarity to the other gibbons they encounter might discourage individuals from breeding. 

A male popped up in an area where we knew there weren't supposed to be any gibbons

Whatever the reason, it creates a problem: if more and more gibbons become isolated, the population won't grow. To save them, we will need to work out why they are so reluctant to form new groups.

It's not a simple – or a cheap – puzzle to solve: finding an answer requires more manpower, better monitoring, and an increase in the use of acoustic technologies to listen in on the gibbons' calls. 

This summer, Turvey and colleagues did just that – with interesting results. For instance, it was previously thought the gibbons fell into three main social groups, with a few solitary individuals. Acoustic monitoring has now made it clear that there is a fourth group. In addition to monitoring their forest calls, the team trialled a method where a gibbons' own recorded call was played back.  

It had the desired effect. "A male popped up in an area where we knew there weren't supposed to be any gibbons." That this fourth social group exists was a welcome discovery. It might even indicate some lone individuals had formed a group after all.

The discovery of this new group raised the number of gibbons from 25 to 26 individuals, says Turvey. The parents may even be previously unaccounted for individuals, which could raise their number to 28.

Another positive sign is the very fact that such a small population has managed to live in the same small patch of forest for 30 years. "It's kind of a fluke that they haven't gone extinct already," says Turvey. 

With so few individuals, a random event could easily have wiped them out: one disease or a freak typhoon would be enough. But, of course, such an event could still occur: to save the species, time is of the essence.

Primates do have the ability to bounce back from small numbers

Fortunately, it's not all doom and gloom for the gibbons. The Primate Specialist Group at the International Union for Conservation of Nature, (IUCN) has recently published its latest edition of the world's 25 most endangered primates – a report that's updated every two years.

It is hoped it will help spread awareness of some of the lesser known primate species around the world – including Hainan gibbons – that are critically endangered. Others at risk include Northern sportive lemurs, (Lepilemur septentrionalis), of which only 50 remain. There are also new additions to the list, including the Philippine tarsier (Tarsius syrichta) and the Lavasoa Mountains dwarf lemur (Cheirogaleus lavasoensis)

Christoph Schwitzer, director of conservation at Bristol Zoological Society, was one of the primatologists involved in the report. He remains hopeful that many of these primates can be saved from oblivion.

With so few remaining, the gibbons are clearly at a higher risk than some of the other critically endangered species – apes need large habitats and are slower to reproduce. So by their very nature it makes them more vulnerable.

But one staggering fact remains. No primate species, that Schwitzer knows of at least, has gone extinct this century or last. "Primates do have the ability to bounce back from small numbers."

Schwitzer is more optimistic for the Hainan gibbons' plight than for some of Madagascar's threatened lemurs. That's because the latter live in a country with an unstable government where forests are frequently targeted by loggers. "The Chinese government on the other hand, are really trying to make an effort," he says.

"I am more hopeful for primates than for some of the other creatures that share their habitats. If we all pool together we can save them. If it was too late there wouldn’t be any good in writing these reports."

Melissa Hogenboom is BBC Earth's feature writer. She is @melissasuzanneh on Twitter. 

Follow BBC Earth on FacebookTwitter, and Instagram